1. Field of the Invention
The present invention relates to a DC-to-DC converter and an electronic device using the DC-to-DC converter.
2. Description of the Related Art
FIG. 10 is a circuit diagram of a conventional DC-to-DC converter. In FIG. 10, the DC-to-DC converter comprises a DC power supply Vcc, an inductance element L1, a diode D1 as a rectifying element, a transistor Q1 as a switching element, resistors R1 and R2, capacitors C1 and C2, a drive circuit 2, a reference voltage generating circuit 3, an error amplifier circuit 4, a triangular wave generating circuit 5, a PWM comparator 6, and an output terminal Pout.
The DC power supply Vcc is connected to one end of the inductance element L1, and the other end of the inductance element L1 is connected to the collector of the transistor Q1 and the anode of the diode D1. The cathode of the diode D1 is connected to the output terminal Pout. The emitter of the transistor Q1 is grounded. The capacitor C1 is connected in parallel to the DC power supply Vcc. The capacitor C2 is coupled between output terminal Pout and ground. The output terminal Pout is grounded through a series circuit comprising the resistor R1 and the resistor R2 in this order. The connection point of the resistors R1 and R2 is connected to one input terminal of the error amplifier circuit 4. The reference voltage generating circuit is connected to the other input terminal of the error amplifier circuit 4. The output of the error amplifier circuit 4 is connected to one input terminal of the PWM comparator 6. The triangular wave generating circuit 5 is connected to the other input terminal of the PWM comparator 6. The output terminal of the PWM comparator 6 is connected to the drive circuit 2, and the output terminal of the drive circuit 2 is connected to the base of the transistor Q1.
The DC-to-DC converter 1 constructed in this way is a step-up DC-to-DC converter, and the transistor Q1 is subjected to on-off control by the drive circuit 2. The current flowing through the inductance element L1 is controlled by the transistor Q1. That is, while the transistor Q1 is turned on, the energy of the DC power supply Vcc is charged in the inductance element L1, and, while the transistor Q1 is turned off, the energy is discharged through the diode D1 and output from the output terminal Pout. The value of the output voltage to be output from the output terminal Pout is determined by the ratio between an ON period and an OFF period of the transistor Q1.
The control of the output voltage is described with reference to FIG. 11. If the ratio between the ON period and the OFF period of the transistor Q1 is constant, the output voltage of the DC-to-DC converter 1 varies when the voltage of the DC power supply Vcc and the load connected to the output terminal Pout changes. Therefore, control is required so that the output voltage may not vary even when the voltage of the DC power supply Vcc and the load change. The output voltage is detected by using the resistors R1 and R2 and is input to the error amplifier circuit 4. A reference voltage generated in the reference voltage generating circuit 3 is input to the error amplifier circuit 4, and an error output a in accordance with the difference between them is output. The higher the output voltage, the higher the error output a. The error output a is input to the PWM comparator 6. The triangular wave output b output from the triangular wave generating circuit 5 is input to the PWM comparator 6, and, comparing both, the PWM comparator 6 outputs a comparison output c that, when the error output a is higher, the comparison output c becomes a LOW level and, when the triangular wave output b is higher, the comparison output c becomes a HIGH level. The time dependence of the error output a, the triangular wave output b and the comparison output c is as shown in FIG. 11, and their relationship is such that the higher the error output a, the longer the period where the error output a is higher than the triangular wave output b, and the duty ratio of the comparison output c decreases. In contrast with this, the lower the error output a, the shorter the period where the error output a is higher than the triangular wave output b, and the duty ratio of the comparison output c increases. The comparison output c is input to the drive circuit 2, and determines the duty ratio in the on-off control of the transistor Q1 by the drive circuit 2. The output voltage increases when the duty ratio becomes larger and the ON period of the transistor Q1 is lengthened, and the output voltage decreases when the ON period is shortened. In this way, when the output voltage becomes higher, the duty ratio in on-off control of the transistor Q1 becomes smaller to decrease the output voltage, and, on the contrary, when the output voltage becomes lower, the duty ratio becomes larger to increase the output voltage, and accordingly the output voltage is controlled to maintain a fixed value.
In the DC-to-DC converter 1 shown in FIG. 10, because the error amplifier circuit 4, the triangular wave generating circuit 5, the PWM comparator 6, etc., are included, the size of the circuit becomes large and a problem occurs in that reduction in cost becomes difficult. Furthermore, since the size of the circuit is large, the DC-to-DC converter is often assembled by using ICs, and in that case, as the size increases and the cost rises, there is a problem in that reduction in cost and also reduction in size becomes difficult.
It is an object of the present invention to solve the above problems and to provide a DC-to-DC converter in which size and cost can be reduced, and an electronic device using the DC-to-DC converter.
In order to attain the above and other objects, a DC-to-DC converter of the present invention comprises an astable multivibrator provided with a first time constant circuit for setting an OFF period of an output and a second time constant circuit for setting an ON period of the output; a switching element controlled by the output of the astable multivibrator; a rectifying element connected to the switching element and an output voltage control circuit for controlling the output voltage such that at least either an ON period or an OFF period of the switching element is changed by changing the time constant of either the first time constant circuit or the second time constant circuit in accordance with the output voltage.
Furthermore, in a DC-to-DC converter of the present invention, an inductance element for charging and discharging energy is provided in series with the switching element.
Furthermore, in a DC-to-DC converter of the present invention, at least one of the first and second time constant circuits is provided with a variable impedance circuit for changing the time constant.
Furthermore, in a DC-to-DC converter of the present invention, a totem-pole circuit is provided between the output of the astable multivibrator and the switching element.
Furthermore, in a DC-to-DC converter of the present invention, the rectifying element is a rectifying switching element, and the astable multivibrator is provided with a first output for on-off controlling the switching element and a second output which is reversed with respect to the first output and which causes the rectifying switching element to turne on while the switching element is turned off.
Furthermore, in a DC-to-DC converter of the present invention, the rising edge of the first and second output waves is tilted or sloped such that the switching element and the rectifying switching element are alternately turned on with periods before being turned on when both elements are turned off.
Furthermore, in a DC-to-DC converter of the present invention, a totem-pole circuit is provided between the output of the astable multivibrator and the rectifying switching element.
Furthermore, in a DC-to-DC converter of the present invention, the rectifying switching element may comprise a MOSFET.
Furthermore, in a DC-to-DC converter of the present invention, the switching element may comprise a MOSFET.
Furthermore, in an electronic device of the present invention, one of the above-described DC-to-DC converters is used.
In a thus constructed DC-to-DC converter of the present invention, reduction in size and cost can be realized with a simple circuit.
Furthermore, in an electronic device of the present invention, reduction in size and cost can be realized with a simple circuit.